In modern times, weight training has been accepted as a method of increasing muscle size and enhancing athletic performance. Generally there are two types of weightlifting equipment in use today: isotonic (equal tension) type devices, such as plate-loaded equipment like the common barbell, and isotonic or isokinetic (variable resistance, equal speed) type selectorized weightlifting equipment, wherein a stack of weights is operatively connected to a pulley or lever to be operated by the user.
The weight on a barbell is typically changed by the addition or deletion of weight plates to each end of the bar prior to lifting. Selectorized equipment generally contemplates the lifting of a preselected portion of the stack of weights can be lifted during each exercise by means of inserting a key at the appropriate position in the stack. This plate-loaded weightlifting equipment and most types of selectorized weightlifting equipment is "isotonic"; that is, the resistance against which the user must push (or pull) remains constant throughout each repetition and is essentially equal to the amount of weight selected. While many of the isotonic resistance mechanisms and equipment have been effective in increasing muscle size and strength, considerable time and effort must be expended to develop a given muscle through its complete range of movement. It has been found, however, that muscle development may be enhanced if the resistance against which the user must work varies in the course of each exercise repetition.
Most selectorized weightlifting equipment has the disadvantage that the user must push or pull against a resistance through a predetermined path with each repetition. This path is determined by the layout of the device and its dimensions. However, users of different sizes will have different skeletal configurations and angles of leverage. To compensate for these individual differences, a weightlifting apparatus should allow user selection of weight engagement positions. It is difficult to design a device which would allow for adjustability in the path of the users movement in the course of each exercise repetition, without having to resort to some complicated and expensive adjustment mechanism.
Reissue U.S. Pat. No. Re. 31,170 discloses an isokinetic apparatus which increases the resistance during an exercise repetition. Stacked weights are manipulated by the user through a lever arm. The lever arm is operatively connected to the stacked weights by a roller moving on the lever arm to restrict the user's mechanical advantage and increase the effective load. While variable resistance is experienced, the user must move through a predetermined non-linear path. The device cannot be adjusted to account for varying skeletal configurations.
U.S. Pat. Nos. 3,858,873, 3,912,261, 4,200,279, 4,311,305, and 4,387,893 disclose variable-resistance isokinetic weight training mechanisms. In each of these devices, a cammed pulley in the lifting means increases resistance in the course of each exercise repetition. However the construction of these devices causes the user to move against the resistance mass through limited, predesigned paths. These devices also cannot be adjusted to accommodate varying skeletal differences.
U.S. Pat. No. 4,627,615 (to the inventor of the present invention) discloses an apparatus which permits both the increasing of weight resistances and the selection of their engagement positions throughout an exercise repetition. The mechanism comprises a plurality of weight stacks. As the user applies force to a cable lifting means, he upwardly pulls a carriage which is attached to a quantity of weights. Aperture selector posts, slidably disposed through each stack, include a slidable collar which can be positioned to conform to individual skeletal lengths. As the carriage engages each collar and weight stacking sequence, the user experiences a progressive weight resistance during the course of a single repetition.
It is an object of the present invention to provide a weightlifting apparatus which is both "hypertonic", meaning that it provides a progressively-varying resistance in the course of each exercise repetition, and that is also "hyperkinetic", meaning that it allows full freedom of the users movement through each exercise repetition. The path through which the user must work against the varying resistance is determined by the user's individual skeletal characteristics.